TORONTO — A new Canadian study has shed fresh light on a growing problem in the nation's maternity wards: pregnant women whose obesity poses health risks for the fetus that may be as dire as the effects of smoking or drinking.

The swelling ranks of obese, expectant mothers face a heightened danger of miscarriages, stillbirths and premature babies, and more likely to deliver children who develop diabetes, cardiovascular disease and even spina bifida. A study released just this week found obese women were almost 70 per cent more likely than others to have an autistic child.

The reasons for such complications have not been clear, but just-published Canadian research on rats fed a special high-fat diet concludes that obesity undermines development of blood vessels in the placenta, which in turn deprives the fetus of sufficient oxygen. The findings point to the fundamental impact that being overweight has on giving birth, but could also help develop drugs to improve results for obese mothers, the authors say.

"These babies, they now have been programmed in utero," said Dr. Andree Gruslin, a maternal-fetal medicine specialist at the University of Ottawa. "Unless something drastic changes, these babies are going to grow to be obese individuals, and they're going to have hypertension, diabetes, cardiac problems . . . And then it's a vicious circle, because then they're going to get pregnant and so on and so forth. It's a real disaster."

The team behind the research cautions that the study used animal subjects, meaning there is no guarantee the findings would translate directly to humans. They note, however, that the female rat models closely parallel women in the early stages of pregnancy and the study confirms earlier suspicions about what makes obese pregnancy so risky.

"I think it's a very important study," said Dr. Gideon Koren, head of the respected Motherrisk program at Toronto's Sick Kids Hospital, and not involved in the study. "It fits the human experience and, actually, it should serve as a red flag for all of us. This is preventable, we know what causes it, but we don't do enough about it."

Studies suggest up to 23 per cent of pregnant women are clinically obese, accounting for as many as a quarter of the 300,000 births in Canada annually.

"Most clinics now in North America have to be equipped with special beds, special chairs, special examination tables, because it's not infrequent to see a woman with a BMI (Body Mass Index) of 50 or higher," said Gruslin.

And research indicates they bring a host of complications to the birth process. Those include higher risk of miscarriage and stillbirth, as well as having a baby born premature, too small or too large. The children are also more apt to suffer from cardiovascular disease and type-2 diabetes later in life, said Sandeep Raha, a biochemist at McMaster University in Hamilton, Ont., and lead author of the study.

The mother's obesity also appears to boost by two- to threefold the possibility of the child being born with spina bifida, said Koren.

Although the specific outcomes are different, the mother-weight factor is likely as significant a risk as drinking during pregnancy — which can trigger fetal-alcohol syndrome — or smoking, which boosts the risks of conditions such as cleft palate and crib death, said Gruslin. The percentage of obese mothers — not even including those merely overweight — exceeds the 13 to 15 per cent of women who smoke while expecting, noted Koren.

It is difficult to examine why motherhood obesity can be dangerous, because of concerns about harming the fetus through research interventions. Animals used in previous studies were made obese with extra feeding shortly before pregnancy, unlike the many obese pregnant women who have had weight problems most of their lives.

For the study, published this month in the journal PloS ONE, Raha and colleagues used rats that were fed a high-fat diet from birth, mimicking as much as possible the condition of the human mothers.

The high-fat rats were more than four times as likely to have stillborn pups and three times as prone to miscarry as a control group fed a healthier diet, while they had fewer pups per litter, and offspring that weighed less on average.

Testing showed that the blood vessels in the obese rats were less "mature," meaning more likely to have problems like leaking and poor nutrient delivery, and indicated there was less oxygen pumped into their placentas.

"That means the fetus is potentially getting less oxygen, which could be a huge problem," said Raha.

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